Many gold and silver-bearing ores are resistant to conventional cyanidation recovery procedures. These ores are commonly referred to as "refractory" ores. As used herein, the term "refractory ore" refers to an ore in which less than about 80 weight percent of the gold in the ore can be recovered by conventional cyanidation procedures.
Pretreatment of such refractory ores by pressure oxidation to make the ores more amenable to subsequent precious metal recovery is known. Operating conditions for a pressure oxidation process are generally disclosed by Hedley, et al. in U.S. Pat. No. 2,777,764 and U.S. Defensive Publication T104001 of Kunter, et al. A number of modifications of the pressure oxidation process have been reported. For example, Mason, et al. in U.S. Pat. No. 4,552,589 (1985) disclosed the alkaline pressure oxidation of a refractory ore slurry followed by cyanidation and precious metal extraction with activated charcoal. Weir in U.S. Pat. No. 4,606,763 (1986) disclosed a pressure oxidation process which used a first compartment having a specific size range to produce autogeneous oxidation of sulfidic materials.
A summary of the various methods of treating refractory gold ores is provided in a paper entitled "Process Options For The Treatment Of Refractory Gold Ores And Concentrates" by P.G. Mason and P.T. O'Kane, presented at the annual meeting Alberta/B.C. Branch Canadian Mineral Processors Vancouver, British Columbia, Nov. 29, 1985, incorporated herein by reference.
Refractory ores which contain high levels of carbonates present particular problems in recovering precious metals from such ores. As used herein, the term "carbonate" ore refers to a gold-bearing ore normally containing from about 0.05 to about 0.25 ounces of gold per ton, commonly small amounts of silver, and from about 1 to about 9 percent by weight total carbon. Most of the carbon present in the ore is in the form of inorganic carbon compounds, principally metal carbonates such as calcite and limestone. The carbonate content of the ore can range up to about 35 percent by weight (based on CO.sub.3). The non-carbonate carbon content of the ore is usually less than 3 percent by weight.
The carbonate content of an ore is commonly determined by measuring carbon dioxide evolved. A typical procedure is described by E. Huffman, Jr. in "Performance of a New Automatic Carbon Dioxide Coulometer" in the Microchemical Journal, Vol. 22, pp. 567-573, 1977, incorporated herein by reference. These ores normally contain up to about 15 weight percent total sulfur primarily in the form of sulfides of iron, arsenic, and other metals. The sulfide content of an ore can be determined by any appropriate method, for example, that described by J.C. Ingles in "Manual of Analytical Methods for the Uranium Concentrating Plant", Dept. of Mines and Technical Surveys, Mines Branch, Ottawa, Monograph 866, incorporated herein by reference.
Some of the problems associated with the acid treatment of an ore which is high in carbonates is described in the paper entitled "Pressure Oxidation -- A New Tool For Refractory Gold Ores And Concentrates" by L.S. Gormely and P.T. O'Kane, presented at the annual Alberta/British Columbia meeting, Canadian Mineral Processors Vancouver, British Columbia, Nov. 30, 1984. As described in this paper, the pressure oxidation under acidic conditions generates carbon dioxide gas which interferes with maintenance of the desired oxygen partial pressure in the oxidation system. In order to minimize the effect of the carbon dioxide production, it is disclosed that (a) a portion of the gas in the vapor space can be bled to prevent carbon dioxide build-up; (b) the ore can be preleached in sulfuric acid to reduce the carbon dioxide generation in the primary pressure oxidation process; or (c) the ore can be charged to the pressure oxidation process and a large tail gas stream can be recycled through an acid gas scrubber to remove the carbon dioxide. Individually, however, these procedures can result in significant economic penalties in loss of reactants and in added equipment.
Matson, et al. in U.S. Pat. No. 4,289,532 (1981) disclosed an oxidation process for treating carbonaceous ores using an oxidation step with oxygen followed by chlorination. The disclosed process requires the use of an alkaline pH medium. Due to the formation of acidic materials during the oxidation, alkaline material must be added to the process to maintain alkaline pH. This patent discloses that the use of acidic oxidation is unsuitable for treating calcium carbonate containing ores.
Weir, et al. in U.S. Pat. No. 4,571,263 (1986) disclosed a method for treating refractory auriferous sulfidic concentrates using pressure oxidation. The method involves pretreating the concentrate with aqueous sulfuric acid to decompose carbonate and acid consuming gange compounds before the pressure oxidation step. There is no suggestion in this patent of a process for the pressure oxidation of carbonate containing ore.
In view of the foregoing, it would be advantageous to have an acidic pressure oxidation process which minimizes consumption of sulfuric acid and oxygen while allowing substantial recovery of precious metals from the carbonate containing ore.